SH2 domains in non-receptor tyrosine kinases, such as p56lck, represent molecular targets that are central to cellular signaling pathways related to immune response. Based on the essential role of SH2 domains a large number of peptides or peptidomimetic compounds that bind to and inhibit SH2 domains have been synthesized. However, these compounds will typically have poor bioavailability and/or stability due to both their peptidic nature and the presence of phosphotyrosine, or related charged chemical moieties. In the proposed study we will apply computer-aided rational drug design to identify small molecules that inhibit cell signaling via binding to the p56lck SH2 domain and lack the undesirable properties of presently available inhibitors. This will be achieved based on the following specific aims. Specific Aim 1) Identify small molecules with a high potential to bind specifically to the p56lck SH2 domain via three-dimensional (3D) chemical database screening. Simultaneous use of the p56lck, Hck, Fyn, Src, Shc and ZAP-70 SH2 domains will allow for selectivity to be included in the computer screening process. Specific Aim 2) Biological assays of compounds identified in Specific Aim 1 including: 2.i) Initial screening for compounds that inhibit OKT-3 mediated stimulation of cellular protein tyrosine phosphorylation in Jurkat cells. 2.ii) Qualitative and quantitative p56lck SH2 binding analysis of active compounds from 2.i). 2.iii) Test the ability of compounds identified in 2.i and 2.ii to inhibit the biological function of T cells, including IL-2 production and mixed lymphocyte culture assay. 2.iv) Test the ability of compounds identified in 2.i, 2.ii, and 2.iii to inhibit in vivo immune response of delayed- type hypersensitivity (DTH) and anti-type II collagen-induced rheumatoid arthritis in mice. Specific Aim 3) Experimental determination of the specificity for p56lck SH2 domain of the active compounds using comparative binding parameters with the SH2 domains of Hck, Fyn, Src, Shc and ZAP-70. Following completion of the proposed study compounds that inhibit the p56lck SH2 domain will be available as research tools and as lead compounds for the development of novel immunosuppressant drugs.